Many industries utilize film heaters, such as automotive, avionics, displays, art conservation, and similar fields, to ensure stable operation of electronic devices under harsh environmental conditions. For example, many LCD displays are rated to degree 0° C. (32° F.) as a minimum operating temperature. Below that, the response can be very slow, or the display may not function properly. Film heaters can be used to maintain the display at an acceptable operating temperature. Optical transparency of the film heater in the visible light range is a requirement in applications such as automobile windshields, electronic displays, and helmet facial shields, or can provide substantial benefits for applications such as microfluidics polymerase chain reactions (PCR) to enable necessary optical diagnosis, and thermochromic signage.
Transparent film heaters can be generally divided into two categories. The first type of film heaters utilizes continuous conductive sheets as heating elements, such as films made of indium tin oxide (ITO) and other transparent conductive oxides (TCO), metals, polymeric composites with conductive fillers, or graphene, as well as random networks of metal nanowires or carbon nanotubes. Continuous conductive films are also widely used as transparent electrodes in many electronic devices for signal transmission.
A common drawback of film heaters based on continuous conductive sheets is that they suffer from non-uniform power density due to the variation of film thickness at different location, and from voltage drop along the bus bars. ITO films have been the most popular materials used in the continuous film heaters. (For example, see U.S. Pat. No. 5,886,763 to Wolkowicz et al., entitled “LCD heater utilizing z-axis conductive adhesive to attach bus bars to ITO;” U.S. Pat. No. 5,247,374, to Terada, entitled “Liquid crystal display device with common heater between two cells;” and U.S. Pat. No. 5,523,873 to Bradford III et al., entitled “LCD heater with flex circuit buss bars.”) However, ITO films have a number of significant disadvantages including slow thermal response, higher materials cost and limited resources, and fragility due to their brittleness. Moreover, the most common method to fabricate ITO films is vacuum deposition which is both expensive and time consuming.
A variety of other materials have been investigated as alternatives to overcome the limitations of ITO, such as doped Zinc Oxides (or other TCOs), metals (e.g., see U.S. Patent Application Publication No. 2015/0096969 to Uprety et al., entitled “Stack including heater layer and drain layer”); metal nanowires such as silver and gold (e.g., see U.S. Patent Application Publication No. 2016/0060468 to Kim et al., entitled “Aqueous compositions, methods of producing conductive thin films using the same, conductive thin films produced thereby, and electronic devices including the same,” and U.S. Pat. No. 9,165,694 to Garnett et al., entitled “Nanowire apparatuses and methods”); carbon nanotubes (e.g., see U.S. Pat. No. 9,237,606 to Yue et al., entitled “Carbon nanotube sheet heater”); graphene (U.S. Patent Application Publication No. 2014/0017444 to Shimizu et al., entitled “Transparent conductive film, heater, touch panel, solar battery, organic el device, liquid crystal device, and electronic paper”); and polymeric composites (e.g., see U.S. Patent Application Publication No. 2015/0114952 to Tai et al., entitled “Flexible transparent film heater”). However, each of these approaches has inherent limitations, and all continuous films are subject to non-uniform power density as ITO films. For example, carbon-based thin film heaters have moderate sheet resistance, requiring either high voltage or low transparency in order to achieve desirable heating performances.
The second type of film heaters uses conductive wires as heating elements. The conductive wires are either on the surface of, or are embedded in, a substrate. Wires can be formed using several approaches including winding, foil etching, printing, or injection into pre-fabricated channels. They can provide uniform power density, but the wires used in present heaters (for example, wire-wound heaters such as those described in U.S. Pat. No. 8,173,939 to Bobgan, entitled “Thermally conditionable light transmitting subassembly,” and screen printed wires such as those described in U.S. Pat. No. 7,053,344 to Surjan et al., entitled “Self-regulating flexible heater,” and U.S. Pat. No. 7,741,582 to Howick et al., entitled “Heater for automotive vehicle and method of forming same”) are large enough to be easily visible to human observers. The wires used in U.S. Pat. No. 8,173,939 are probably the smallest of the known prior art configurations (25 μm or larger in diameter), but they are still much larger than the size that would be required to be invisible to the naked eye. Ideally, the wire size should less than about 10 μm to be substantially invisible.
Fabrication of conductive wires using foil etching techniques such as that described by Mavraki, et al. in the paper “A continuous flow μPCR device with integrated microheaters on a flexible polyimide substrate” (Procedia Engineering, Vol. 25, 2011, pp. 1245-1248) also involves a costly and time-consuming photolithographic process. In addition, such wires are formed on top of the substrate, creating an uneven surface which is not favorable for subsequent integration such as lamination onto displays.
Conductive wire can be embedded in the substrate by fabricating channels on the substrate and subsequently injecting conductive inks into the channels. In the article “Polydimethylsiloxane microfluidic chip with integrated microheater and thermal sensor” (Biomicrofluidics, Vol. 3, 012005, 2009), J. Wu et al. describe a microheater made of conductive wires formed by billing pre-fabricated channels having a width of 400 μm. Although this approach avoids the protrusion of wires, the wire size (400 μm) is too large for the wires to be invisible to a human observer. Furthermore, the fabrication process is not suitable for large scale manufacturing.
Metal mesh consisting of micro-wires less than 10 μm wide is gaining popularity for use in transparent electrodes for applications such as touch sensors. While such configurations are well-suited for signal transmission as required for touch sensors, the size of the micro-wires is intrinsically limited by the conventional fabrication methods, and therefore they are unsuitable to transmit sufficient power for heater applications.
There remains a need to provide a transparent film heater with invisible micro-wires and uniform power density, which can overcome the aforementioned limitations and can be fabricated using low cost roll-to-roll manufacturing techniques.